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  JVM-11-双亲委派模型
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      <h1 id="JVM-11-双亲委派模型"><a href="#JVM-11-双亲委派模型" class="headerlink" title="JVM-11-双亲委派模型"></a>JVM-11-双亲委派模型</h1><h2 id="前序"><a href="#前序" class="headerlink" title="前序"></a>前序</h2><p>​    在上一篇中，我们介绍了类加载的过程，包括五个阶段</p>
<ul>
<li>加载</li>
<li>验证</li>
<li>准备</li>
<li>解析</li>
<li>初始化</li>
</ul>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200528/100029480.png" alt="mark"></p>
<p>本篇博客，我们来介绍Java虚拟机的双亲委派模型，在介绍之前，我先抛出一个问题：</p>
<p>我们知道，在JDK源码中，有各种Java自带的类，比如java.lang.String，java.util.List等，那么我们自己的项目中，能够写一个命名为java.lang.String.java 等JDK源码中存在的类，并且在项目中使用吗？</p>
<a id="more"></a>

<h2 id="1-类加载器"><a href="#1-类加载器" class="headerlink" title="1. 类加载器"></a>1. 类加载器</h2><p>什么是类加载器？上篇中我们介绍类加载的<strong>第一个阶段–加载</strong>，作用是“通过一个类的全限定名来获取描述这个文件的二进制流” ，那么这个加载过程<strong>就是类加载器来完成的</strong>。</p>
<p>从Java虚拟机角度出发，只有两种不同的类加载器</p>
<ul>
<li>一种是<strong>启动类加载器</strong>（Bootstrap ClassLoader），这个类加载器使用 C++ 语言实现，是虚拟机自身的一部分；</li>
<li>另一种是所有其他类的加载器，这些类加载器都是由Java语言实现的。但是从Java开发人员角度来看，类加载器可以分为以下四种：</li>
</ul>
<ol>
<li><strong>启动类加载器（Bootstrap ClassLoader）</strong></li>
</ol>
<p>负责将存放在 <strong>/lib</strong> 目录中的，或者被<strong>-Xbootclasspath</strong> 参数所指定的路径中的，并且是虚拟机按照文件名识别的（仅按照文件名识别，如rt.jar，名字不符合的类库即使放在lib目录中也不会被加载）类库加载到虚拟机内存中。</p>
<p>启动类加载器无法被Java程序直接使用。</p>
<p>JDK中的源码类大都是由启动类加载器加载，比如前面说的java.lang.String ，java.lang.List等，需要注意的是，启动类 main Class也是由启动类加载器加载。</p>
<ol start="2">
<li><strong>扩展类加载器（Extension ClassLoader）</strong></li>
</ol>
<p>这个类加载器由 <strong>sun.misc.Launcher$ExtClassLoader</strong> 实现，负责加载<strong>＜JAVA_HOME＞/lib/ext</strong> 目录中的，或者被 <strong>java.ext.dirs</strong> 系统变量所指定的路径中的所有类库。</p>
<p>开发人员可以直接使用扩展类加载器。</p>
<ol start="3">
<li><strong>应用程序类加载器（Application ClassLoader）</strong></li>
</ol>
<p>由 <strong>sun.misc.Launcher$AppClassLoader</strong> 实现。由于这个类加载器是ClassLoader.getSystemClassLoader()方法的返回值，所以一般也称它为系统类加载器。</p>
<p>它负责加载用户类路径ClassPath上所指定的类库，开发者可以直接使用这个类加载器。如果应用程序中没有自定义过自己的类加载器，一般情况下这个就是程序中默认的类加载器。</p>
<p>通常项目中自定义的类，都会放在类路径下，由应用程序类加载器加载。</p>
<ol start="4">
<li><strong>自定义类加载器（User ClassLoader）</strong></li>
</ol>
<p>这是由用户自己定义的类加载器，一般情况下我们不会自定义类加载器，但是有些特殊情况下，比如JDBC能够通过连接各种不同的数据库就是由自定义类加载器实现的，具体在后文中会介绍。</p>
<h2 id="2-双亲委派模型"><a href="#2-双亲委派模型" class="headerlink" title="2. 双亲委派模型"></a>2. 双亲委派模型</h2><p>回到文章开头提出的问题，如果有不法分子在你项目中构造了一个java.lang.String类，并在该类中植入了一些不良代码，但你自己浑然不知，以为使用的String类还是 rt.jar 包下的，那可能会给你系统造成不良的影响。</p>
<p>聪明的Java虚拟机实现者也想到了这个问题，于是，他们引入双亲委派模型法来解决这个问题。</p>
<p>下面是双亲委派模型的加载流程机制：</p>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200528/164330884.png" alt="mark"></p>
<p><strong>总结来说：</strong></p>
<ul>
<li><p>双亲委派机制就是如果一个类加载器收到了类加载的请求，它首先不会自己尝试去加载这个类，而是把这个请求委派给父类加载器去完成，每一层次的类加载器都是如此。</p>
</li>
<li><p>因此，所有的加载请求最终都应该传送到了顶层的启动类加载器中，只有父类加载器反馈到无法完成这个加载请求（它的搜索范围没有找到这个类），子类加载器才会自己尝试去加载。</p>
</li>
<li><p>　其实，这里叫双亲委派可能有点不妥，因为按道理来讲只有父加载器，这里的“双亲”是“parents”的直译，并不表示汉语中的父母双亲。另外，这里的父加载器也不是继承的关系。</p>
</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ClassLoaderTest</span> </span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        ClassLoader classLoader1 = ClassLoaderTest<span class="class">.<span class="keyword">class</span>.<span class="title">getClassLoader</span>()</span>;</span><br><span class="line">        ClassLoader classLoader2 = classLoader1.getParent();</span><br><span class="line">        ClassLoader classLoader3 = classLoader2.getParent();</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">        System.out.println(classLoader1);</span><br><span class="line">        System.out.println(classLoader2);</span><br><span class="line">        System.out.println(classLoader3);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>



<p>结果如下：</p>
<p><img src="http://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200528/164956709.png" alt="mark"></p>
<p>那么知道了什么是双亲委派机制，双亲委派机制有什么好处呢?</p>
<p>回到上面提出的问题，如果你自定义了一个 java.lang.String类，你会发现这个自定义的String.java<strong>可以正常编译，但是永远无法被加载运行</strong>。因为加载这个类的加载器，会一层一层的往上推，最终由启动类加载器来加载，而启动类加载的会是源码包下的String类，不是你自定义的String类。</p>
<h2 id="3-双亲委派模型源码"><a href="#3-双亲委派模型源码" class="headerlink" title="3. 双亲委派模型源码"></a>3. 双亲委派模型源码</h2><p>可以打开 java.lang.ClassLoader 类，其 loadClass方法如下：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">protected</span> Class&lt;?&gt; loadClass(String name, <span class="keyword">boolean</span> resolve)</span><br><span class="line">        <span class="keyword">throws</span> ClassNotFoundException</span><br><span class="line">    &#123;</span><br><span class="line">        <span class="keyword">synchronized</span> (getClassLoadingLock(name)) &#123;</span><br><span class="line">            <span class="comment">// First, check if the class has already been loaded</span></span><br><span class="line">            Class&lt;?&gt; c = findLoadedClass(name);</span><br><span class="line">            <span class="keyword">if</span> (c == <span class="keyword">null</span>) &#123;</span><br><span class="line">                <span class="keyword">long</span> t0 = System.nanoTime();</span><br><span class="line">                <span class="keyword">try</span> &#123;</span><br><span class="line">                    <span class="keyword">if</span> (parent != <span class="keyword">null</span>) &#123;</span><br><span class="line">                        c = parent.loadClass(name, <span class="keyword">false</span>);</span><br><span class="line">                    &#125; <span class="keyword">else</span> &#123;</span><br><span class="line">                        c = findBootstrapClassOrNull(name);</span><br><span class="line">                    &#125;</span><br><span class="line">                &#125; <span class="keyword">catch</span> (ClassNotFoundException e) &#123;</span><br><span class="line">                    <span class="comment">// ClassNotFoundException thrown if class not found</span></span><br><span class="line">                    <span class="comment">// from the non-null parent class loader</span></span><br><span class="line">                &#125;</span><br><span class="line"></span><br><span class="line">                <span class="keyword">if</span> (c == <span class="keyword">null</span>) &#123;</span><br><span class="line">                    <span class="comment">// If still not found, then invoke findClass in order</span></span><br><span class="line">                    <span class="comment">// to find the class.</span></span><br><span class="line">                    <span class="keyword">long</span> t1 = System.nanoTime();</span><br><span class="line">                    c = findClass(name);</span><br><span class="line"></span><br><span class="line">                    <span class="comment">// this is the defining class loader; record the stats</span></span><br><span class="line">                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);</span><br><span class="line">                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);</span><br><span class="line">                    sun.misc.PerfCounter.getFindClasses().increment();</span><br><span class="line">                &#125;</span><br><span class="line">            &#125;</span><br><span class="line">            <span class="keyword">if</span> (resolve) &#123;</span><br><span class="line">                resolveClass(c);</span><br><span class="line">            &#125;</span><br><span class="line">            <span class="keyword">return</span> c;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br></pre></td></tr></table></figure>

<p>实现方式很简单：</p>
<ul>
<li>首先会检查该类是否已经被加载过了，若加载过了就直接返回（默认resolve取false）</li>
<li>若没有被加载，则调用父类加载器方法，若父类加载器为空则默认使用启动类加载器组作为父类加载器，如果父类加载器失败，则抛出ClassNotFoundException 异常后，使用自己的<code>findClass</code>方法进行加载</li>
</ul>
<h2 id="4-自定义类加载器"><a href="#4-自定义类加载器" class="headerlink" title="4. 自定义类加载器"></a>4. 自定义类加载器</h2><p>先说说我们为什么要自定义类加载器？</p>
<ol>
<li><strong>加密</strong></li>
</ol>
<p>我们知道Java字节码是可以进行反编译的，在某些安全性高的场景，是不允许这种情况发生的。那么我们可以将编译后的代码用某种加密算法进行加密，加密后的文件就不能再用常规的类加载器去加载类了。而我们自己可以自定义类加载器在加载的时候先解密，然后在加载。</p>
<ol start="2">
<li><strong>动态创建</strong></li>
</ol>
<p>比如很有名的动态代理。</p>
<ol start="3">
<li><strong>从非标准的来源加载代码</strong></li>
</ol>
<p>我们不用非要从class文件中获取定义此类的二进制流，还可以从数据库，从网络中，或者从zip包等。</p>
<p><strong>明白了为什么要自定义类加载器，接下来我们再来详述如何自定义类加载器。</strong></p>
<ul>
<li>通过上面第三小节的java.lang.ClassLoader 类的源码分析，类加载器根据双亲委派模型辉县一层层找到父类加载器，如果加载失败，则会调用当前加载器的<code>findClass()</code>方法来完成加载，因此<strong>我们自定义类加载器，需要两个步骤</strong></li>
</ul>
<ol>
<li><strong>继承 ClassLoader 类</strong></li>
<li><strong>重写findClass() 方法</strong></li>
</ol>

      
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